The present invention relates to a device for the spotwise imaging of printing surfaces with the aid of at least one laser beam which is moved relative to the printing surface.
During the imaging of printing plates in CtP (computer-to-plate) or direct-imaging printing machines, the spacing between the printing surface and the optical system of the imaging device has to be maintained very accurately to obtain an optimum result. However, deviations from the intended distance between the printing surface and the imaging laser arise, for example, because of oscillations of the machine during operation. The extent to which the quality of the imaging result depends on the deviation from the intended distance is determined, inter alia, by the beam quality of the laser and the selected beam parameters. A deviation from the intended distance generally gives rise to a deformed printing spot which is either larger or smaller than the predefined nominal size results form, depending on the beam parameters. In the case of very large deviations, even no printing spot is generated at all on the printing surface because the laser beam is widened to such an extent that the imaging threshold is no longer reached at any location of the printing surface.
U.S. Pat. No. 5,764,272 discloses an autofocus system for a laser imaging device. This system has a laser and a corresponding optics for forming a light beam which is focused on an image plane. Via a photodiode, a signal which is characteristic of the light reflected from the image surface is generated so that the focus of the laser beam on the image surface can be correspondingly adapted to the characteristic signal. In this manner, a close association of the image surface and the image plane of the laser including its corresponding optics is brought about. For shifting the focus of the imaging device, it is possible to move the laser, the corresponding optics or the image surface.
Autofocus systems of this kind can work only at limited speeds. For example, if the laser optics is moved, it is required for a mass that is not negligible to be quickly accelerated, accurately positioned, and quickly decelerated again. For high-frequency disturbances such those that arise, for example, due to dirt accumulations under the printing surface, dust particles or because of folds in the printing surface, the control times needed by such a system are too long. Therefore, imaging defects occur frequently. In a multichannel system, i.e., an imaging device having a plurality of parallel laser beams, it is typically not possible to focus each individual beam since the whole imaging optics is moved. In other words: a compromise must be found so that the deviation from the intended distance of all simultaneous beams altogether becomes minimal. Generally, the design of a mechanical autofocus system which functions by moving the imaging optics requires considerable technical outlay, a corresponding constructional space, and causes a relatively great expense.
An object of the present invention is to provide a device for the spotwise imaging of printing surfaces with the aid of at least one laser beam which is moved relative to the printing surface and which makes it possible to carry out a variable imaging without having to mechanically move parts of the device such as the imaging optics to compensate for variations in the distance between the imaging optics and the printing surface.
This objective may be achieved by a device for the spotwise imaging of printing surfaces with the aid of at least one laser beam which is moved relative to the printing surface, wherein a laser control (426) is included which varies the laser power or the exposure time as a function of the distance of the laser light source (40) from the image spot (410).
The present invention also provides a method for the imaging of printing surfaces with the aid of at least one laser beam comprising the steps of:
providing a laser light source (40) for generating a laser beam (42) having a position-dependent intensity distribution in the two spatial directions perpendicular to the propagation axis, and a specific divergence;
providing a printing surface (48) at a distance from the laser light source (40);
exposure of the printing surface (48) located at a certain distance from the laser light source (40); characterized by
the variation of the laser power or exposure time for varying the spot size of image spots (410) on the printing surface (48).
The present invention in addition provides a method for generating printing spots of desired size comprising the steps of: providing a laser light source (40) for generating a laser beam (42) having a position-dependent intensity distribution in the two spatial directions perpendicular to the propagation axis, and a certain divergence; and providing a printing surface (48) at a distance from the laser light source (40); characterized by
the measurement of the distance of the laser light source (40) from the printing surface (48); and
the adjustment of the spot size to a predetermined value by varying the laser power or exposure time.
The imaging optics of an imaging device is typically adjusted in such a manner that, at the intended distance, the focus, i.e., the plane in which the laser beam has its smallest diameter comes to rest exactly on the surface of the printing surface. A deviation from the intended distance between the laser and the printing surface results in an increase in the beam diameter on the printing surface and, consequently, in an increase or reduction in size of the printing spot, depending on the adjustment of the laser parameters of power and focus diameter. The actual distance between the printing surface and the laser is measured by means of a detector so that it can be compared to a setpoint value. The optical power used for imaging is increased or reduced as a function of the deviation from the setpoint value. An increase in the laser power is associated with an increase in size of the printing spot since the spot size on which energy exceeding the imaging threshold is deposited on the printing surface increases. Correspondingly, a reduction in the laser power is associated with a reduction in size of the printing spot since the spot size on which energy exceeding the imaging threshold is deposited on the printing surface decreases.
A further way of varying the size of the printing spot is to selectively prolong or shorten the exposure time. A combination of the change in the power and in the exposure time is also possible.
Using the device according to the present invention, the increase or reduction in size of the printing spot due to a deviation in distance can be compensated for: via the provided variable laser power, it is possible to adapt the printing spot size so that an acceptable imaging result is attained. In other words: the printing spot size is variable. The value of the required optical power or exposure time can be computed from the measured distance. This function can be carried out, for example, in the raster generator which converts the printing spot pattern to be imaged into a time sequence of pulses for the laser imaging. In an advantageous manner, a table, a so-called xe2x80x9clookup tablexe2x80x9d, is prepared and stored in the preliminary stages via the functional relation so that the required value is immediately available in situ.
In an advantageous refinement of the present invention, the device for the spotwise imaging of printing surfaces has a plurality of laser beams which are used for simultaneous imaging. In this context, in particular individually controllable diode laser arrays are given preference. The power or the imaging time can be varied for each individual laser of the array, making it possible to attain an acceptable imaging result since the size of each printing spot written by a laser is variable and independent of the size of the other printing spots.
The present invention requires considerably fewer moving parts than the known autofocus systems and can therefore react much more quickly to disturbances. At the same time, it attains a markedly better imaging result than a device without autofocus. The implementation of compact imaging devices in an integrated form is markedly easier. It involves lower cost.
A device of this kind can be used inside or outside of a printing unit or a printing machine for spotwise imaging.